Exemplary power devices include power modules (power supply units) for converting power from AC to DC in information appliances such as personal computers, OA appliances and household appliances, and power devices for converting power from DC to AC for driving induction motors and the like in electric automobiles and hybrid automobiles. Such a power device generally includes a power semiconductor device, and a highly heat-conductive heat sink substrate having a mount surface on which the power semiconductor device is mounted for transferring heat generated by the semiconductor device during operation thereof.
A Si semiconductor device is generally used as the power semiconductor device. The Si semiconductor device typically includes a Si chip including a semiconductor circuit formed in a Si monocrystalline substrate thereof, and a circuit substrate having interconnections and the like for connection to the Si chip with the Si chip being mounted thereon. The circuit substrate has an insulative property, and is formed of AlN or the like which has a relatively high thermal conductivity.
Usable as the heat sink substrate on which the Si semiconductor device is to be mounted is a metal substrate such as of Cu, Al, Cu—Mo or Cu—W which has higher thermal conductivity and ensures easier production of a large-area heat sink substrate than AlN. A resin adhesive, a solder or the like is used for bonding each member mentioned above. In recent years, the power devices are required to have a higher output. However, the conventional Si semiconductor device is unlikely to meet this requirement.
To meet the aforesaid requirement, a SiC semiconductor device including a SiC chip having a SiC monocrystalline substrate formed with a semiconductor circuit is under development as an alternative to the Si semiconductor device for practical use as a power semiconductor device. SiC is about three times greater than Si in band gap. This physical property advantageously imparts the SiC semiconductor device with higher breakdown voltage and capability of operating at higher temperatures with a lower loss. Particularly, the SiC semiconductor device is stably operable at a temperature of 250 to 300° C. Therefore, the use of the SiC semiconductor device conceivably makes it possible to significantly increase the output of the power device as compared with the Si semiconductor device.
Since the SiC semiconductor device has a high operating temperature as described above, the heat sink substrate is required to have:
(a) a higher thermal conductivity as described above; and
(b) a coefficient thermal expansion close to that of SiC (4.2×10−6/K) for relaxation of a thermal stress during operation of the semiconductor device. However, the conventional metal substrate has a coefficient thermal expansion of not less than 5×10−6/K, which is significantly different from that of SiC. Therefore, the conventional metal substrate fails to meet the requirement (b) and, hence, is not suitable as the heat sink substrate for the SiC semiconductor device.
To this end, a Si—SiC composite is under development as a heat sink substrate material which has a coefficient thermal expansion close to that of the SiC semiconductor device. Known examples of the composite are Si—SiC composites prepared by a reaction sintering method (E. Scafe et al., “Thermal Diffusivity of Silicon-Silicon Carbide Composites”, ADVANCED STRUCTURAL INORGANIC COMPOSITES, P. Vincenzini (Editor), published by Elsevier Science Publishers B. V., 1991). However, a size of the Si—SiC composite product produced by the reaction sintering method is limited. Therefore, it is difficult to produce a larger area heat sink substrate from the Si—SiC composite by the reaction sintering method.
To cope with this problem, there are proposed heat sink substrates each formed of a Si—SiC composite prepared by a so-called melt-infiltration method (see WO00/076940A1 and JP2004-281851A). Namely the Si—SiC composite is prepared by forming a three-dimensional network structure of SiC ceramic, infiltrating thermally melted Si melt into pores of the network structure, and cooling the resulting structure to solidify Si, and at least a surface of the resulting Si/SiC composite structure later serving as a mount surface of the heat sink substrate is polished as required.
The size of the Si—SiC composite structure prepared by the melt-infiltration method is defined by the size of the three-dimensional network structure. The three-dimensional network structure is formed as having a desired size, for example, by mixing ceramic powder containing SiC powder with an organic binder, forming the resulting mixture into a predetermined shape, and firing the resulting product to remove the organic binder and sinter the ceramic powder. This makes it easy to produce a heat sink substrate having an increased surface area.
A research conducted by the inventors shows that the conventional Si—SiC composite heat sink substrates produced by the melt-infiltration method described in the above patent publications satisfy the requirements (a) and (b), and are each allowed to have an increased surface area as described above. However, the efficiency of heat conduction from a semiconductor device such as a SiC semiconductor device mounted on the mount surface is lower. Therefore, these heat sink substrates are revealed to fail in providing a sufficient transfer of heat effect.